The U.S. TV programme came to an end after 15 seasons on Thursday night (19Nov20), [SPOILER ALERT!] with Jensen's character Dean dying as he got impaled on a piece of steel while fighting a group of vampires.
During a chat with Entertainment Weekly, Jensen revealed he and co-star Jared Padalecki, who stars as Dean's brother Sam in the show, learned of their characters' fate when they visited the writers room ahead of starting production on season 15.
"Yeah, so, during the hiatus between season 14 and 15, Jared and I went out to LA and sat down in the writers room, which I think is the first time we've ever done that," he said. "It wasn't like a brainstorming session. It was like, this is what we have, take it or leave it. And I just walked out of there kind of uneasy. I don't know if it was just the fact that I just heard the ending of a show that had been going for 15 years and I'm just too close to it to really accept a finality to it."
"It certainly wasn't settling in as easy as I kind of hoped. I struggled with it."
But a conversation with series creator Eric Kripke helped Jensen come to terms with the ending.
"He just kind of put things in a perspective that really helped me kind of identify why I was having trouble with it," he mused. "But also why it was a really great idea for an ending. And so from then on, I was kind of all in."
The ending caused huge shockwaves from viewers on social media, many of whom expressed their outrage after a character who had survived so much in the 15 seasons was killed in the final episode.
"The writers are going to HELL for this! #Supernatural," one person wrote, while another added, "jensen was so right for hating this episode we should've listened #supernatural."
A third commented, "idk wtf is going on in supernatural rn but it rlly shouldve ended at season 5 (sic)."